Hollow waveguides present an alternative to solid core fibers at the infra-red (IR) regime where suitable optical materials are scarce. In addition, due to their air core, they can be used for broad-spectrum high power transmission as they suffer from small insertion losses. As a result, hollow waveguides are used in industrial and medical applications involving CO2 and Er:YAG lasers as well as for spectroscopic and radiometric measurements. In 1897, Lord Rayleigh was the first to consider using hollow metallic waveguides for the propagation of electromagnetic radiation. However, he considered the metal to be a perfect conductor, thus his solution is inadequate in optical regimes where metal behaves more like a lossy dielectric. In 1961, Snitzer presented a general treatment for the propagation of electromagnetic fields inside cylindrical waveguides of arbitrary material. A convenient approximation for circular metallic waveguides was later presented by Marcatili and Schmeltzer in 1964. In 1984, Kawakami and Miyagi proposed an improved design in which an additional inner dielectric multilayer stack is used to reduce transmission losses. Recently, a new design for a circular hollow Bragg waveguide, which is a type of photonic band gap fiber, has been presented [S. Johnson, M. Ibanescu, M. Skorobogatiy, O. Weisberg, T. Engeness, M. Soljacic, S. Jacobs, J. Joannopoulos, and Y. Fink, “Low-loss asymptotically single-mode propagation in large-core OmniGuide fibers,” Opt. Express 9, 748-779 (2001), and B. Temelkuran, S. D. Hart, G. Benoit, J. D. Joannopoulos, Y. Fink, “Wavelength-scalable hollow optical fibres with large photonic bandgaps for CO2 laser transmission”, Nature 420, 650 (2002)]. In this case, guiding is obtained by reflection from a periodic dielectric layers rather than by metal cladding. Therefore, the waveguide performance is no longer limited by the metallic losses.
A hollow waveguide supports vectorial modes in a manner determined by its structure and material composition. It is customary to classify the modes: transverse electric-TE0m, transverse magnetic-TM0m, and hybrid-HEnm, and EHnm-modes. The integers n, m>0 denote the azimuthal and radial mode orders. The commonly used modes of circular hollow waveguides are the TE01 and HE11; The TE01 is an azimuthal linearly polarized vectorial vortex having a dark central core. This mode possesses the least amount of loss in a bare circular metallic hollow waveguide as well as in the circular hollow Bragg waveguide; The HE11 mode is linearly polarized and has a bright central core. It is the lowest order mode in terms of waveguide cutoff. It is important to note that a general hollow waveguide mode has a spatially varying polarization state, with the exception of the linearly polarized HE1m set of modes.
While current applications of hollow waveguides, such as power delivery, rely on multimode operation, future applications might benefit from the ability to excite only a single waveguide mode. Such applications include hollow waveguide lasers and single TE01 mode circular Bragg waveguides. We expect that additional applications of hollow waveguides might emerge once higher order modes are exploited. For example, the mode's dark core increment with azimuthal mode order n might prove useful for blue detuned atom guiding. Also higher order modes can be used for dispersion compensation.
Coupling a single hollow waveguide mode requires matching the phase, amplitude, and polarization state. Phase and amplitude matching methods are well developed and can be achieved by conventional optical devices, irregular waveguides, and diffractive or holographic optics. However, matching the polarization state is more challenging. Several techniques for this purpose exist such as liquid crystal spatial light modulators, interferometric techniques, and lasers with intra-cavity optical devices. However, all these methods are either cumbersome, have low power thresholds or inadequate in the IR regime.
Recently, we have demonstrated spatial polarization state manipulation by space-variant subwavelength gratings [E. Hasman, G. Biener, A. Niv, and V. Kleiner, “Space-variant polarization manipulation,” in Progress in Optics, vol. 47, E. Wolf ed. (Elsevier, Netherlands, Amsterdam, 2005)]. These devices act as waveplates with space-variant orientations and as inhomogeneous anisotropic subwavelength structures, they are particularly well suited for polarization manipulation. As the optical properties of these devices stem from the geometric Pancharatnam-Berry phase, they are called Pancharatnam-Berry phase optical elements (PBOEs). PBOEs are both compact and efficient optical devices. They were used for the formation of propagation invariant vectorial Bessel beams [A. Niv, G. Biener, V. Kleiner, and E. Hasman, “Propagation-invariant vectorial Bessel beams obtained by use of quantized Pancharatnam-Berry phase optical elements,” Opt. Lett. 29, 238-240], rotating vectorial vortices [A. Niv, G. Biener, V. Kleiner, and E. Hasman, “Rotating vectorial vortices produced by space-variant subwavelength gratings,” Opt. Lett. 30, 2933-2935 (2005)], and for the excitation of a vectorial hollow waveguide mode in the 1.55 μm wavelength regime [W. S. Mohammed, A. Mehta, M. Pitchumani, and E. G. Johnson, “Selective excitation of the TE01 mode in hollow-glass waveguide using a subwavelength grating”, Photon. Technol. Lett. 17, 1441 (2005)]. Furthermore, we have presented the use of PBOEs for the coupling and inverse coupling of free-space linearly polarized beams to a hollow waveguide's azimuthally polarized vectorial TE02 mode [Y. Yirmiyahu, A. Niv, G. Biener, V. Kleiner, and E. Hasman, “Vectorial vortex mode transformation for a hollow waveguide using Pancharatnam-Berry phase optical elements,” Opt. Lett. 31, 3252-3254 (2006)].
It is a purpose of the present invention to provide a general approach for coupling free space beams to any of the hollow waveguide modes, thus enabling single mode operation.